Radioactive exploration



Oct. 16, 1956 w. M. STRATFORD 2,

RADIOACTIVE EXPLORATION Filed ma 21, 1952 a Sheets-Sheet 2 IN VEN TOR.Wu, 1. MM M STP/l TFORD ATTORNE Y5 Oct. 16, 1956 w. M. STRATFORD 2,

RADIOACTIVE EXPLORATION Filed May 21, 1952 5 Shets-Sheet 5 INVENTOR.

Wu. 4. {A M M 5777A TFOPD 'ATTOENE)? United States Patent Q iRADIOACTIVE EXPLORATION William M. Stratford, New York, N. Y., assignorto Texaco Development Corporation, New York, N. Y., a corporation ofDelaware Application May 21, 1952, Serial No. 289,091

7 Claims. (Cl. 256-816) This invention relates to prospecting formineral deposits in the surface of the earth, and more particularly toprospecting by radioactive detection.

This application is a continuation-in-part of my U. S. patentapplication Serim No. 62,347 which was filed on November 27, 1948, nowabandoned.

The principle of radioactive detection has been utilized in the past inprospecting for strongly radioactive minerals such as those containinguranium or radium, to provide direct and positive indications of exposeddeposits thereof. In such prospecting, portable Geiger-Mueller countersare moved along the ground surface in regions in which experienceindicates the possibility of such deposits, and the counting rate isobserved. The presence of a strongly radioactive deposit, if it is notburied too deeply, will be evidenced in some such way as by an increasein the detectors counting rate. Since this prospecting is inpredetermined localized regions the detection apparatus can be carriedon the backs of the operators or on slow pack animals. Where a depositis buried beneath a large amount of overburden, so much of the radiationemitted by it may be absorbed that significant variations therein areobscured.

In the U. S. Patent 2,562,914 which issued to Gerhard Herzog on August7, 1951, a method is disclosed wherein such deeply buried deposits, andeven deep deposits of certain non-radioactive ores, can be locatedindirectly by detecting significant variations in the intensity of gammaradiation emitted by the overburden (or country rock) rather than by thedeposit itself, preferably by the use of radiation detectors, such asthose of the multiple-plate type (see Hare Patent No. 2,397,071, issuedMarch 19, 1946), or scintillorneters, or groups (banks) thereof, whichhave gamma ray counting efficiencies well in excess of those ofconventional Geiger- Mueller counters.

Whether the detector means or head used in either of the above-mentionedforms of prior art prospecting comprises a single detector or aso-called bundle thereof, it customarily is sensitive to radiationapproaching it from a large, not precisely-defined zone, such as a largearea of the earths surface surrounding a vertical projection thereon ofthe vehicle carrying the detector, and it provides but a single recordof intensity variations. Its non-directional sensitivity results fromthe fact that'customarily the detector is exposed, as indeed it may bewithout detriment to accomplish the purposes for which it is intended insuch types of prospecting, to all radiation approaching it from theearth over a very large solid angle, such as one of substantially 2 pi.Its inability to provide more than a single record results from the factthat, even when a number of detectors (a so-called bundle), is used,they are not individually shielded to prevent radiation which floods oneof them, after approaching it over a given solid angle, from alsoflooding one or more of the others, e. g., they all receive radiationfrom the same area of the earths surface; Accordingly it has beencustomary to connect them all to a single recorder.

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The present invention is concerned with using radioactivity prospectingfor other purposes in addition to locating specific deposits withinregions previously localized in other ways, such as by miningexperience, geological studies, etc., and its value in all suchprospecting is based on recognition of the limitations of the prior artpractices of using non-directive detectors and of making only onerecord.

One particularly important purpose for which radioactivity prospcctingcan be used is to obtain data from which one may plot contour lines orisoradins designating regions on a map where natural emissions ofradiation have discrete respective densities, e. g., average, x% aboveaverage, y% below, etc. This information can be correlated with otherkinds of information available to geologists to help them in localizingregions in which detailed prospecting may be carried out in any suitableway, for example by the two kinds of radioactivity prospecting firstmentioned above. While this kind of information can certainly bedeveloped for localities containing areas of highly concentratedradioactivity, due, for example, to deposits containing uranium and/orradium, it can also be developed for areas in which the only radiationencountered is the weak natural radiation emitted by the traces ofradioactive materials which are so sparsely distributed everywhere inthe earths constituents that they are not otherwise detectable. Moreoverwhile this information will certainly be helpful in localizing regionswherein strongly radioactive deposits are likely to occur (and perhapsit may even serve to pin-point the deposits themselves), it will also behelpful, though in a less obvious Way, in localizing regions which arelikely to contain non-radioactive minerals such as metallic ores,petroleum, etc.

However in order to obtain this kind of information the detector ordetector means whether it be of the single or multiple detector typemust be able to perceive, -i. e., resolve, radioactivity anomalies whichmaybe manifested by small-magnitude intensity-changes and may occur inrelatively small areas of the terrain being surveyed. Such resolutioncannot be achieved by successively exposing the detector to radiationemanating from large areas of the earths surface since a small-magnitudeanomaly occupying a small area cannot appreciably increase or diminishthe total radiation from a large area including it. Thus the presence ofa physically invisible geological fault, which could be evidenced by asmall but definite difference in the intensities of natural gammaradiation emitted by the earths solids on each side of the fault, mightpass undetected if the prospecting is done with a detector whichreceives all of the radiation frrom a large area at any one instant.

Incidentally since this type of radioactivity prospecting is in thenature of a reconnaissance to be conducted over wide areas, it cannot berapidly practiced by using an operator or a slow-pack animal forcarrying the detector. prospecting should be carried out as an aerialsurvey. However, as the detector is carried to greater and greaterheights, the ground area from which radiation approaches it over a fixedsolid angle of exposure rapidly increases while at the same time theradiation is attenuated en route in accordance with the inverse squarelaw.

Accordingly radioactivity prospecting as it has been practiced in thepast is not to be preferred for this survey type of radioactivityprospecting since the firstof its above-mentioned limitations makes itdiificult to perceive, i. e., to resolve, radiation anomalies whichconsist of small magnitude changes in the intensities of the radiationemanating from small adjacent areas on the ground. Incidentally it isnoted that the perception of small-area anomalies may have great value,in addition to its value Preferably though not necessarily this type of'stricted-to a small-solid angle having-a ape v detector means and itsbase on "a ground area small 7 with a detectonshoWn moving thereover.

inthe preparation of contour maps of theltind described above, asdirectclues to the presence of'types'of stron'gly radioactive ore bodieswhich, may often extend for considerable distances at easily workabledepths below the surface buthave .one or perhaps a-fevv-surfa'ceoutcroppings which are so small as to be detected. only byapparatushavinggood resolving power. p

Therefore "according to one'oftheieatures of the present'invention thedetectors zone of sensitivityis rex near th enough to afford therequired resolving-1; \ovv e'r. I

However*tvvopractical difficulties are'e'ncountercd in carrying out airbo'rne radioactivity prospecting withfa f he aircraft 7 detector havingdirectional sensitivit must 'fiy' a-greatly increased nutribr V a giv'enarea since the strip erare terrain co vered in flying a -siri'gle courseis 1 greatlyfrduced; (2) it'is much more difiicult fo'r'the pilotflyi'n' successiveeourses 'to.

maintain parallelism with the same percent'age ot curacy; and (3)-changing aspectsofthe -plane dne to bankingfclirnbing,desc'ending,;e'tc. wilhswing'the detectors'lobe offlsensitivity awa rmmaeanansmwahme ground'thus altering the lobes and the length of theradiation' patha-long it frorn'thefground; A'ccordingly if is aprincipal object of'this'invention V to {provide a "novel method I of{radioactivity prospecting whereby anomalies in the surface'of'theearth,'even of very limited areas, may more readily be determined.

Anether-'--object "is to provide radiation detection apparatus 'capableof "detecting small-area "radiation anomalies at the-surfaceof the eartheven when the apparatus isairbor'ne and operating at relatively highaltitudes.

:lt is still another object to'provide radiation detectionapparatus'vvhichmay have directional sensitivity and be' transportedaboard an aircraft Without incurring the three difiiculties mentionedabove.

s'till a further object, particularly in aerial surveys, is to avoidanyrestriction to a limited range of altitudes andlto'enab'le such'surveysto be conducted at any reasonable altitudes while still maintainingthedesireddiscrimination'with'respect to's'mall areas of the ground,'thustakingadvantage-of detectors of increased sensitivities.

.tlie' last difliculty 'isaverted by using flexible mounting fer eaehdetector in such a way that, "despite tilting "and dlpping of theplane-and'the like, the detectors narrow lobe of sensltivity 'is'alw'ayspointing in the same downward dire'c't-ion.

In -the dra'wing: Fi'g. l is a'horizontal eetionjtaken-throug'h apreferred detector assembly with the associated 'amplifyingand recordingapparatus shown diagrammatically.

'Fig. 2 is a front-elevation of the detectorassembly of Fig. l,thedetecto'rs therein being'shown in broken lines.

Figf-Efrepresents-aportion of a surface beingsurveyed Fig.4is*aiportion-ofarecord made by the detector assembly of Fig.3. 'Figf'Scorresponds to Fig.3 eXcept'that a different detector assembly is shown.

6 is a portion'of arecord made-by the detector assembly Of Fig-LS.

Fig. 7 is a diagrammatic showing of a modified detector-"assembly.

Fig. 8 is a perspective of'another modified form of detector assemblywith the amplifiers shown diagrammati cally. 7 a

Figs. 9 and' 10 are diagrammatic representations of modified detectorarrangements, and

Fig. 11 represents anembodime nt of the present invention in which thecollimating'shielding means are of f of dead weight burden atype whichimposes a minimum on the aircraft. p

In'one broad aspect, the.invention'contemplatesrprospecting ofthesurface of the earth by measuring substantially independently theradiationema'nating'from a num- I ber of limited-size areasof'thatsurfaceand received at a corresponding number of respectivepoints which have constant spaced-relationships to one another but areall moving relative to the surface of the earth, and correlating themeasurements so obtained "to determined anomalies -in"thc prospectingarea. More "specifically, the invention involves the substantiallyindependent-measuring of radiation, such asgamma radiation received at anumber-of-respective points in fiXedspaced-relationship toone anotherand moving-with respect to the prospected'area, the radiation measuredat'each point being taken rinsefar -as possible from a relatively smallportion of the prospecte'd area. a V

-The term substantially independent as used'herei'tr is to b'edistinguished from any sense of absolute-independency. This may beexplained-by restating the well knownfact that such detectors may beaffected by radiation from all surrounding points, such as by cosmicradiation air' contamination, etc. In the present case therefore it iscontemplated as preferable that the'detector assembly as a Whole beshielded as by suitable metal shielding insofar as possible fromtheundesirable radiation c'omingfrom points other than the'area beingexplored;'and th'at each individual detector, where a plurality ofdetectors is use'd in one assembly, beindividually shielded so that asource of radiation which floods one of them will be unable to flood anyother, and'preferably also'so asfto exclude cosmic radiation and other'undesired radiation insofar as possible so that there remains only aso-called openorfsensitive'face through Whicha principal aXis'of theindividual lobe of sensitivity'of that detector 'canbe said to'pass andto intersect the particular area under-its observation. By usingshielding between the'detectors,-the-radiation gvvhich has'e'nteredo'necan be prevented fromreaching'anyother whereby the individual detectorscan be said to'besubstantially :i'n'dependent'of one another. V V

The-invention can-best be'explained byexample. *Assume that "are1atively-la rgesection of the *ear'ths surface is to be prospected. Inthe methods proposed'heretofdfe-involvingradiation detection, detectorassemblies chai'aeterized by their exposure to all of the "radiationcoining from' relatively large areas at any --g'iven instant havebeenernployed. In each unit of time which'elapses while such a detectorassembly is moved over the earths siirfa'ce, the assembly receivesradiation from a total area which includes whatmay be termed a "-steadyarea, this beingalarg'cintermediate area which is constantly withinthe-Zone"of sensitivtiyof'the'detector during all (if the interval; anincrement of area-which progressively comes"withinthe'zone ofse'nsiti'vity, i.'e., is"gained,.' in the direcfion of movement of thea'ssembly during the interval; andan incrementofarea which is lost in theopposite direction during the-interval, both of said inereinentsbeing'ver'y 'smalllpor'tions of said total area. In recordingtheradiation detected in one interval .of time from such a: total area,the assembly cannot. perceive. the presence thereinofan-increment'which-is significantly more orless-radioactive"than What isnormal for-the. area. This is'becausethe ,detector'willintegrate theradiation from the steady area, the increment :being gained, and theincrement being lost. Thus if one increment is low in intensity it maybalance another increment which is high in intensity with the resultthat the total observed intensity may be the same as that of adjacentareas wherein all increments are of equal intensity. Thus an areacontaining an anomaly, either positive or negative, may be overlooked.In other words, not only does the detector integrate all of theradiation emanating from the large area to which it is exposed at anyinstant but in addition it integrates the radiation from the even largerarea to which it is exposed during a finite interval of time while it isin motion.

From the above, it might be concluded that any attempt to observe moreclosely the radiation from ground areas should be accompaniednecessarily by a corresponding decrease in the rate of progress of sucha survey. Contrary to this supposition, the present invention provides amethod wherein the rate of progress of such a survey can be maintainedwhile insuring the resolution of substantially all anomalies of possibleinterest.

In a preferred embodiment of the invention, this is accomplished byemploying a detector assembly including a plurality of detectors, eachdetector being substantially independent of the others, and all beingarranged in a definite fixed relationship to one another. Such anassembly is shown in Figs. 1 and 2 wherein four detectors 11, 12, 13 and14 are shown mounted in one assembly, the assembly being mounted in anaircraft with the exposed ends of the detectors facing downwardly asshown in Fig. 2. The assembly is shielded from side and top radiationincluding cosmic radiation by a shield 15 extending around the detectorbundle and a cap 16 extending across the top thereof. Verticalpartitions 17 and 18 function to shield the detectors so that the sameradiation which enters any one of them Will be unable to reach anyother. The net result is a detector assembly composed of substantiallyindependent detectors, the principal axes of sensitivity of thedetectors being substantially parallel and extended downwardly as viewedin Fig. 2. By varying the length of the open-ended cylinder formed byshield 15, the ground area subject to observation by each detector canbe varied correspondingly. For example, if shield 15 as viewed in Fig. 2be increased in length, the extent of the ground area subject to eachdetector will be reduced. If the length of shield 15 be progressivelyreduced, without reducing the lengths of the vertical partitions 17, 18,the axes of sensitivity of the detectors will become progressively moredivergent and the extent of the ground area under observation by each ofthem will be progressively increased. Ease and flexibility of adjustmentof the elfective length of the shield may be afforded in a variety ofways such as by providing a longitudinally slidable sleeve 15a aboutshield 15 and by mounting the partitions so that they are verticallyslidable in the sleeve 15.

Each detector is provided preferably with its own preamplifier inconventional manner and connected to its own amplifier as shown at 11a,12a, 13a, and 14a, each amplifier in turn being connected to recordingapparatus of well-known type such as that employing an electronicvoltmeter wherein the intensity variations detected by each detector canbe recorded individually as by recorders 11b, 12b, 13b and 14b on asuitable record, the tracings being subsequently correlated.

Figs. 3 and 4 illustrate a typical survey, Fig. 3 being a plan view of aportion of the earths surface. The prospecting is carried out by movingthe detector assembly D, as in a helicopter, along predetermined linesof survey such as lines 19 and 2% An ore body having a typicalhorizontal outcrop A is shown. In proceeding along lines of survey 19and 29 and using a detector assembly of the type wherein radiation froma large area is totaled, it is entirely possible that the anomaly incident to the ore body may be overlooked. The survey on line wouldmiss theanomaly entirely. In the survey on line 19, the radiation from thesurface surrounding outcrop A might be of such low intensity as tobalance any increase in intensity from the outcrop. Or if the ore bodyhe of a non-radioactive mineral, i. e., a negative anomaly, the conversemight be true.

In surveying the same area by the method of the presentinvention,detector assembly D carrying a plurality of substantially independentdetectors 1, 2, 3 and 4 will be moved along the same survey line 19 inthe direction indicated. By reason of the relatively small area of thesurveyed surface from which radiation is passing to each detector, ascontrasted to the size of the area supplying radiation to the previouslydiscussed detector assembly, significant variations in the intensity ofradiation are recorded as the detector assembly passes over outcrop Aand the anomaly is betrayed.

This may be accomplished by recording the readings of detectors 1, 2, 3and 4 separately as shown in Fig. 4. The pip on the trace of detector 1shows clearly the change in intensity caused by the anomaly and alsoindicates the probable extent of the anomaly by reason of the degree ofthe pip size. This is corroborated by the trace of detector 3. The traceof detector 2 with its,

pip denotes a variation of less intensity and shorter duration, thisbeing verified by the trace of detector 4. Thus independent traces areprovided by each area and it is possible to determine not only theexistence of the anomaly but also its probable outline. This can be doneby spacing the base lines (quiescent or zero-input base lines) of thetraces of the laterally adjacent detectors by an amount proportional tothe spacing between the principal axes of sensitivity of the detectorsand accordingly proportional to the spacing between the centers oftherespective terrain strips from which those detectors receive radiation.Thus traces 1 and 2 will be proportionally spaced from one another aswill traces 3 and 4. If lines T1 and T2 be drawn through thecorresponding base ends of the pips as shown in Fig. 4, the result is afigure generally corresponding in outline to that of the anomaly. Whilethe two detectors, 1 and 2, in adjacent relation may be sufiicient todetermine the anomaly, it is desirable to check or verify as byadditional detectors 3 and 4.

It will be noted that all the above is accomplished by restricting thearea from which the individual detectors receive radiation so that anyappreciable increase or decrease in radiation intensity of anyincrements of 'appreciable size will be effective on the total radiationreported by the detector. Stated otherwise the area from which eachdetector receives radiation is kept relatively small to substantiallyeliminate any possibility of its having incremental parts which canbalance out one another with opposite variations from normalcy in the intensities of their emitted radiations.

Once the existence, locations and general outline of an anomaly isdetermined, geologists may determine the significance thereof and thearea can be further prospected as by boring and to more exactlydetermine its value. Where the radioactivity anomaly corresponds to anoutcropping of an ore body, particularly if the ore contains a stronglyradioactive material such as uranium, such further prospecting wouldobviously lead to the discovery of the main ore body.

Fig. 5 illustrates a different detector arrangement D' wherein theindividual detectors are arranged on an axis generally transverse of theline of survey. In moving on line of survey 19 in the directionindicated, the detector assembly moves across outcrop A, the intensityvariations measured by each detector being recorded as traces on thesegment of the chart shown in Fig. 6. It will be noted that the pip intrace 1 indicates the edge of the main ore body as well as the base ofoutcrop A, the width of the base of the pip generally corresponding tothe width of the base of the outcrop. The tracesof detectors 'sensitiverde'tectorsare used. j a

21. enilviindi ate a iat ons o ess xten a d .5

. duraticn, V

-11! b notedcfrom; Fig. 6that by drawing lines. T3; .35151 n e t ilgv eor po d na of he. as s at the intensity variations asmanifested-bythepips, a rough" outline of the outcrop can be made. As already statedthis is -hest' accomplished. by. spacing the traces; apart in amountsapproximatelyproportionahto the; spacing be- 'aetiyity prospecting; asdescribed herein, i. e., prospecting whiohnuses aplurality' ofcollimated'; detectors, may also beieondllctedi on the surface of theearth, or even undergnou-nd. in mining shafts .as by an individualcarrying the 7 equipment pack-style or by mounting the equipment onamotor. vehicle, thedetectors being set up preferably in amanner-toiscan strips of the terrain on opposite sides of thevehicleasit moves along a line of survey. Where suchisurveys are tobe, conductedabove the ground, as byihelicopter, titudes between 100 and, 200 ft.,preferably. ab.Q.11t..150fi., are satisfactory. However operationsrelatively. small areas of earth surface despite increases in. altitude.This becomes increasingly important; asmore zRreliminary reconnaissance.surveys can plishedihythe method disclosed herein by using detectors,such as anassembly thereof which are exposed to radiationfrornrelativelylarge areas on the earth?s surface, i. e., detectorswhich are notextremely directive. In such a case,ithe gr.ound can be'surveyedpreliminarily with such detectors and thereafter the more interestingareas may be more'carefully'prospected by more stringentapplicationzofftheimethod; of: this invention.

While the cor-.ventional Geiger-Mueller counter may be used in somecases, itis preferredto useother and more sensitive types .of detectorsespecially. where natural gammaradiationis to be' determined. Faintdifierences in natural gamma ray intensities have frequently beenusefullylindicative ofdeeplyburiedore deposits as well as of the. linesofdemarkation ofacontacts, faults and other geologic features.Accordingly :it is preferred to use detectors having higher'efliciencies for gamma rays than 7 that some types of multiple-anodedetectors, ordinarily considered single detectors, may be used providingthey be appropriately modified to function as a plurality ofsubstantially"independent detectors. Referring to Fig- 8, the detectorshown therein diagrammatically is formed of-aplurality of spaced cathodeplates 22, the several plates'being'formed' with a series of alignedholes 23 through which fine wires 24- forming anodes are passed. Thedetector is-dividedinto a plurality of separatesectors on'ehambers. asby one or more shields Z .and 26, ar.-. ranged in the drawing to .formquadrants as respects. theca er Y s be accom-.

may bereadily conducted at higher altitudes and significant readingsobtained because of the ability, by collimating' one:or- .allof thedetectors-to confine their exposures to In thi -p ys rra sem v n w h.epa a eke-1:

trical output circuits for the respective. anodes,the ;foun detectorsthus formed can be said to be substantially; independent .of Ofi?another, the manner, offtheir; performance being entirely .oomparable'tothe detectorar rangementshown in Fig; 1. t The four anode wires are:respectively connected to suitable-preamplifiers and separate amplifiers27, 28, 29 and 30- and recording apparatus of the type shown in- Fig.2so that-the several traces canbe individually' recorded in the; samemanner as shown in Fig. l, the operationjof'the'deyice being the samedescribed in connectionjwith 'Figsu 3 and ltisto be understood that thecathode platescan be arranged in other ways such as those shown in theabove identified patent and-that a single partitioningelement .25 or-adifien.

ent, plurality thereoimay be used to vary the number ofsnbstantiallyindependent detectors, The-'apparatus-shown in Fig; 8 is also shownanddescribed in co-pen'ding application, Pat. No. 2,699,513 which wasfiled'on, March. 18, 1950, the said apparatus being claimed-in thatapplica tion and notherein,

In some instances, it may be desirable to vary the. distance betweenground areas under observation by, thev individual detectors. This-maybe done by mounting the detectors as shown-in Big. 9wherein twodetectors 41 and 42 are shownito be angularlymovable about a pivot 43,,which is suitably mounted on a. support such as'a heli-i.

copter; By'adjusting' the detectors in a manner to change the anglebetween them, for example as shown on a quadrant, thedistanee betweenthe ground areas on the. surface 7 under, obsenvation by each detectorcan'be adjusted pro! portionally.

In: the arrangement shownin Fig. 10,,detectors 44 and, 45 -are mountedonlateral.supports and 47 on ahelirc opter. Each detector is mounted onarack or equivalent;

means-so that it can-bead-justedalong itsssupport to and from the bodyof the=helicopteru Thus by moving the detectors-to the positions shownin dotted lines, the ground;

areas; under observation are brought closer together. (Bymovingthedetectors outwardly, the ground areas are moved further apartr Theembodiment shown inFig. 1.1 is arranged to maintainconstant the downwarddirectivityofa detectorslobe of sensitivity. ln addition'itutilizesatype' of collimating' shield. which imposes. a minimum of deadWeight burden.

detectors. However, if-preferred it may have anyone of: anumber-ofditferent shapes,.e. g., it may be-cube' or-slab shaped orit'mayhave-thesame cylindrical shape as shown herein but be mounted with itsaxis in a horizontal position rather than standing on end, and insteadof being a single detector it may consist of a plurality thereofcompactly arranged-in abundle and electrically connected in parallel.Moreover, as pointed out above, it may be a scintillometer and as suchbe of any suitable geometry and positioned many-suitable Way. Forany'particu'largeometrical shape of the detector 59 a shield which isintended to collimate upward radiation. so that the detector will seeonly (t-predetermined small ground area must have aradiation-impermeable portion'which lies between the radiation sensitivebottom surface of the detector'and is the lowenpor-tion thereof whichextends downward belowthe lower-periphery of the detectors 1114.

' will beexpla'ined below, its upper portion, i. e., the upper half ofthe-shield l5 and the cap 16, .actually'are not-f V necessaryzfor.collimating the ground emanations, though, to besuretheyareaveryusefulrfor -.excluding.side.and top;

radiation such as "radiation coming from air radioactivity and fromouter spaces respectively. Not only must the portion which has thecollimating effect with respect to ground emanations lie between thebottom of the detector and the ground but in addition it must be moreand more distant from the bottom of the detector as the aircraft mountsto higher and higher altitudes and as the exposed bottom area of thedetector is greater and greater in size. Thus the type of collimatingshield which is shown in Fig. 2 may sometimes have to be of considerablelength, and therefore present a very undesirable dead weight burden, ifa high degree of resolving power is to be maintained even though (1) asurvey is to be conducted at the fairly high altitudes, and (2) a largearea detector is to be used (to make up for the fact that the fluxdensity of natural gamma radiation will be very low at such altitudes).

However, this is not true of the type of shielding arrangement shown inFig. 11. This arrangement makes use of the fact that radiation whichleaves the earth in the direction of the detector but from points atsubstantial distances from the point on the ground directly beneath thedetector will have such long slant paths of travel to reach the detectorthat they will often be absorbed without the use of shielding. Sincethis is so, very effective collimation can be attained by blocking-offonly those slant paths which extend to the detector from a limitedregion, e. g., the annular region 51 shown in Fig. 11, surrounding thesmall ground area, 52, under surveillance directly below the detector.

The outer periphery of the region 51 may be determined in practice asthe boundary beyond which gamma rays directed toward the aircraft havesuch long slant ranges that no more than a negligible percentage of themwill be able to reach it. Accordingly all that is necessary is that thecollimating shield intercept all radiation paths from the ground to thedetector which lies outside of the imaginary cone 53 representing theintended lobe of sensitivity for the detector and inside of the conedefined by imaginary lines, 54, extending between the outer periphery ofthe region 51 and the detector 50. Accordingly such a shield may simplytake the form of a flat plate, say of lead, which is circular in shapeand has a round hole at its center. Moreover if, like the plate 55 inFig. 11, it has the shape of a section of a sphere whose centercoincides with that of the detector 50 it will have even less deadweight, this being also true to an approximation if it has the shape ofan apertured inverted frustum so shallow as to be tangential to thelower surface of the shield 55. inasmuch as the slant ranges of gammarays become progressively longer as they emanate from parts of theregion 51 progressively nearer to its outer periphery, and sincetherefore the amounts of air-absorption and distanceattenuation (inaccordance with the inverse square law) will also increaseprogressively, the shield 55 may be made thinner near to its outerperiphery than near its center opening without any substantial sacrificein its effectiveness.

The detector 50 is mounted in a gimbal arrangement 56 having reverseaxes 56a and 56b. Thus the detector 50, the rods 57 and the shield 55constitute an assembly articulately coupled to the aircraft at a pointon the assembly which is well above its center of mass with the shield55 suspended below it so that its weight can serve to maintain downwardorientation of the directivity of the detector. To prevent the airstream from tilting back the detector away from the direction of flightit may be covered with a windshield in the form of a radiationpermeablecanopy 58, the whole being suitably mounted on a vehicle such as ahelicopter. While they are not shown in this figure it is intended to beunderstood that the usual output circuit components will be employed inconjunction with and suitably connected to the detector 50.

Obviously, if desired, a more complicated arrangement including one ormore gyroscopes and a servo system '11) might be employed formaintaining downward orientation in lieu of the simple gravity-actuatedarrangement of Fig. 11.

Where it is desired to reduce the background counts to a minimum, topand side shielding of the body of the detector may be used inconjunction with a disc-type shield, like the shield 55 and in manyinstallations the total weight of the shielding will still be less thanthat of a corresponding shield of the kind shown in Fig. 2.

Where it is possible to control the flight stability to such an extentthat there is no necessity of using means for maintaining downwarddirectivity for the detectora simple structural arrangement which mayprove to be suitable will be that of mounting the detector near a topportion of the air frame of the craft such as near the ceiling of thefuselage or the top surface of the wing while an apertured collimatingplate is fastened to a part of the air frame directly below such as tothe floor of the fuselage or the bottom surface of the wing. In such anarrangement, or in the arrangement of Fig. 11 with its lightweight tierods 57, heavy material is used only where it serves a definiteshielding function.

While only one collimated detector or detector means is shown in Fig.11, it is intended that according to the present invention a number ofsuch detectors or detector means may be used for example in the mannershown in Fig. 10. Moreover, if it be desired to mount such detectorsmore closely side by side, as in Fig. 9, the Fig. 11 arrangement forcontrolling the directivity of the detector may be adjusted so as totilt the detectors lobes of sensitivity slightly away from one anotherand to thereafter maintain them in that orientation. T 0 this end aslotted weight 59 is provided which may be slipped over the protrudingbottom end of any one of the tie rods 57; fastened thereto; and adjustedthereon to automatically control the exact downward direction of theassociated detectors lobe of sensitivity.

The degree of collimation afforded by the arrangement shown in Fig. 11can be adjusted by moving the plate 55 upwards or downwards on the tierods 57 and fixing it at any position so selected by tightening setscrews in retaining means such as the collars shown at 60. Whererelatively large adjustments are contemplated a number of differentplates 55 of different sizes may be made available and the rods 57 mayhave their upper ends artic lately connected to the detector 50.

Tests of the present invention have shown it possible to locateradioactivity anomalies with collimating shielding which could not belocated without it.

Obviously many modifications and variations of the invention, ashereinbefore set forth may be made without departing from the spirit andscope thereof, and therefore only such limitations should be imposed asare indicated in the appended claims.

I claim:

1. In geophysical operations involving the measurement of gamma raysemitted by the earth along a traverse of its surface, the improvementwhich comprises the steps of detecting radiation from selected areas ofthe earth surface by moving a plurality of detector units over andrelative to said surface, said units being substantially shielded fromone another and from radiation other than from the earth surface andarranged in closely adjacent and fixed relation to one another, saidunits being positioned in a line substantially normal to said line oftraverse whereby the radiation measured by each unit at any instant issubstantially from a different area of said surface, which areas liealong a line substantially normal to said line of traverse, andseparately and continuously recording the measurements from said unitsWhile correlating said measurements with respect to the movement of saidunits relative to the earth surface.

2. The improvement of claim 1 wherein said detector units are moved in aplane substantially parallel to the earth surface whereby the areascanned by each detector 71*1 En ire shape f airihhonr Qfeuhstantiallyuniform. wi th; and substantially par lel said; line, of raver e 3.Geophysical apparatus for the measurement of; gamma'raysemitted by theearth-alongla' traverse-of its surface, the apparatus being mountfld Qua-aircraft and comprising a plurality of detector units.v for detectinggamma radiation-from selected areas of the earth surface, -said detectorunits being substantially individually shielded from radiation otherthan from respective .discrete areas. of the earth surface andbeing-anrm ged m fixed relation to one another, said units being;positioned in a line substantially normalito said line of traverse,wherebythe radiation measured by each :unit at any instantissubstantially from-a difierent area of said sur face which .areastliealong; a line substantially normal to saidline of traverse, andmeansttor separately and continuous'ly recording and correlating the,measurements 7 from said units on a single tape, whereby the apparatuscan provide plural and correlated indications of a common geophysicalanomaly. r

4 Geophysical apparatus for the measurement of gamma rays emitted :bythe earthalong a traverse, of its surface, theapparatus' beingrnountedon aniaircraft'and comprising a plurality of detector units fordetecting gamma radiation from selected areas of the earth surface, saiddetector, units being substantially individually shielded from radiationother thanffrorn fispfictive discrete areas of the earthsurface and.being arranged in fixedjrelation to one another, said unitsbeingpositioned in a line substantially normal to said line of traverse,

whereby the radiation measured by each un'it at any 7 instant issubstantially from a difierent area of said sur-] face which areas liealong a line substantially normal to said line of traverse, and meansfor separately and continuously recording and correlating themeasurements from said units, whereby the apparatus can providepluraland correlated indications of a common geophysical anomaly. I V

5."Geopnysical apparatus for the measurement of gamma rays emitted bythe earth along a traverse of its surface, the apparatus being mountedonan aircraft and comprising a plurality of detector units for detectinggamma radiation from selected areas of the earth surface, said detectorunits being substantially individually shielded from radiation otherthan from respective discrete areas of the earth surface-and beingarranged in fixedrelatjon to on -ano er, atleast two lorfsaidunits:

heingpositioned, on opposite sides of a line parallel to the line; oftraverse, whereby the radiation measured, by

said twounits is from difierent areas which .areas are displaced; fromone another in adirection substantiallynormal to the line of traverse,and means .for separately and continuously recording and correlating'themeasure-7' mentstfrorn said units, whereby theapparatus can provide"plural and correlated indications, of a common geophysi: caLanomaly. t

'6. in geophysical operations involving the measurement-of gamma raysemitted by th'e earth along a trav-g erse'of its surface, theimprovement which comprises the steps of detecting radiationfromselectedareas of the earth-surface by moving a pluralityof detectorunits over and relative to said surface, vsaidgunits being substantially shielded from one another and from radiation other than-fromthe earth surface and arranged iniclosely adjacentand fixed relation toone another, at least two of said units being positioned on oppositesides of a line parallel to the line of traverse, whereby the radia-,tion measured bysaid two .units is-from difierent areas which areas aredisplaced from one another in a direotion substantially normal to theline of traverse, and

meansfor. separately and continuously'recording and correlating themeasurements .from said units, whereby the apparatus can provide pluraland correlated indications of a common geophysical anomaly.

7; The apparatus of claim 4 which further comprises means for mountingsaid apparatus on an aircraft 'for universal movement relative to,the'aircraft in order to maintain the detector apparatus in verticalalignment duriug flight of said aircraft.

References Cited-,in-the'file of this patent UNITED STATES PATENTSVol.27, 1936, PP. 149-I157.

